This code is weird. It reads back the transition that it just set from
the shared internals. It's almost like it expects it to be a getter or
something.
This avoids that and makes it consistent with what ReactFiberHooks
already does.
DiffTrain build for commit https://github.com/facebook/react/commit/15da9174518f18f82869767ebe2a21be2fc8bd90.